1. Field of the Invention
The present invention relates to fiber optic gyros and more particularly, to a rate extension improvement to fiber optic gyros with digital control.
2. Description of the Prior Art.
In a paper entitled "INTEGRATED OPTICS : A PRACTICAL SOLUTION FOR THE FIBER-OPTIC GYROSCOPE" BY H. C. Lefevre, S. Vatoux, M. Papuchon and C. Puech presented to the 10th Anniversary Conference 1986 of SPIE and printed in SPIE Vol. 719 Fiber Optic Gyros on pages 101-112, a digitally controlled closed loop fiber optic gyro is discussed primarily on pages 103 and 104. In such a closed loop system, as will be explained below, the system supplies a re-balance voltage in the form of a ramp voltage (occurring in a series of steps) to a phase modulator which operates on the counter-rotating beams to introduce a differential phase shift to the light beams which phase shift counterbalances the phase shift which is due to the rotation being measured. The consequences of this action is observed with a photo-diode detector which provides the feedback signal. To prevent too much build up of voltage into the modulator, the stepped signal ramp is reset toward zero by 2.pi. radians of the light phase as needed. Under normal circumstances, this makes no difference since the photo-diode detector provides the same output for a differential phase shift angle 2.pi. + .alpha. as for an angle .alpha.. A difficulty with this approach is that there is a rate limit beyond which more than one 2.pi. resets is needed to keep the modulator voltage within range during a single step. This limit occurs at a value of 2.pi./SF radians/second where SF is the scale factor relating the Sagnac phase shift to the input rate of the gyro. Above the 2.pi./SF rate limit, the prior art system cannot properly determine rates.